1. Field of the Invention
This invention relates to attenuator circuits, and more particularly to attenuator circuits that employ resistor ladders to provide selectable attenuation levels.
2. Description of the Related Art
Variable voltage attenuation circuits are employed in applications such as audio recording and playback. Perhaps the simplest form of a variable or selectable attenuator is a chain of series resistors, with taps taken between successive resistors in the chain. Due to the different impedance at each tap, the tapped voltage signals require buffering before they can be connected to additional attenuation circuits that may be desired to provide a wider attenuation range. The buffer circuits, however, add undesirable levels of noise, voltage offset and gain errors, and require relatively large amounts of area.
One form of a series resistor chain is employed in the Toshiba Corporation TC9213 attenuator, which provides a logarithmic attenuation. To cover the relatively large attenuation range, parallel resistors of relatively higher resistance values are used to implement the low effective resistances required at the lower end of the chain. The use of higher resistance value elements in parallel facilitates the achievement of more accurate low resistance values, However, buffer amplifiers with their attendant disadvantages are still required between successive attenuation circuits.
Another way to achieve a variable attenuation is to control the gain or attenuation setting of a logarithmic voltage controlled amplifier (VCA) with a linear digital-to-analog converter (DAC). If the DAC output is negative, the circuit will operate as an attenuator. This approach is illustrated, for example, in the Precision Monolithics, Inc. Analog Integrated Circuits Data Book, volume 10, 1990, page 11-260 in connection with the DAC-8800 converter and the SSM-2014 VCA. However, the required circuitry is considerably more complex than for a simple resistive voltage divider.
DACs themselves commonly use R-2R resistor ladders for transforming a digital signal into an accumulation of binarially-weighted bits to produce an output analog signal. An R-2R ladder consists of a chain of resistors with resistance value R in series, with each series resistor shunted by a resistor of value 2R. With the addition of a termination resistor of value 2R after the last series and shunt resistors, the input resistance at each step of the ladder is equal to R. The use of an R-2R resistor ladder in a current-mode configuration is described in Grebene, Bipolar and MOS Analog Integrated Circuit Design, John Wiley & Sons, 1984, page 758-760 and 773-777, while a voltage-mode DAC configuration is shown, for example, on page 11-202 of the Precision Monolithics, Inc. Data Book mentioned above in connection with the PM-7226 Quad No. 8 Bit CMOS DAC. Each of the shunt resistors in the voltage-mode design is switched into or out of the ladder, depending upon the input digital signal. The ladder functions in effect as a digitally controlled variable resistor, but does not provide an accessible voltage decremention.